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McDonough testified that her former boyfriend, Seth Mazzaglia, strangled Marriott from behind with a rope while the three of them were watching a movie and after Marriott had said it would not be OK for McDonough and Mazzaglia to have sex in front of her in their disheveled, dimly lit studio apartment.

“That’s when he moved up behind her and he pulled the rope over her neck, and he strangled her,” McDonough testified at Strafford County Superior Court. “She was sitting right next to me.”

Mazzaglia, 31, faces charges of first- and second-degree murder in the Oct. 9, 2012, death of Marriott in the Sawyer Mills studio he shared with McDonough, 20.

In a subdued, quavering voice, McDonough said that as Mazzaglia strangled Marriott with a white, cotton rope while wearing black leather gloves, she numbly rose and moved to one of the two windows in their Sawyer Mills studio.

“I glanced over and saw it happen, and then I glanced back up at the TV and tried to focus my thoughts, because I didn’t know what to do,” she said. “I got up to the window, and I shut the first window, and when (Mazzaglia) gestured, I shut the second window.”

McDonough said Marriott “almost immediately” stopped moving after Mazzaglia began strangling her, and gave only a “short…quick gasp.”

McDonough is serving a 1½- to three-year prison term after pleading guilty last July to charges including witness tampering and hindering the investigation. She testified Tuesday that she helped cover up the murder of Marriot, whose limp body she said Mazzaglia then raped.

In last Wednesday’s opening statements, defense attorney Joachim Barth told jurors it was McDonough, not Mazzaglia, who killed Marriott that night after a game of strip poker and the following requests for participation in sexual activities.

Prosecutor Peter Hinckley pressed McDonough on Wednesday morning about her actions during and after Marriott’s death, specifically her failure to call police or for an ambulance, and her decision to remain in the apartment and not seek help.

“I didn’t have anywhere else to go,” McDonough said. “I loved him, I didn’t want to lose him, he was the only real person I had. …Part of me considered leaving. I didn’t know where I would go and I didn’t know what I would do.”

After she closed the windows and curtains, McDonough said, she went into the bathroom, the studio’s only other room. When she came out, she said, she saw Mazzaglia raping Marriott, who was lying on her back, limp, on the floor.

McDonough said Mazzaglia fondled Marriott and insulted her as he raped her for several minutes before rising.

“He told me to hold the rope back over her neck while he went to the bathroom,” she said. “The whole thing was so unbelievable and so shocking, that it was just easier to do what he said.

“I was telling him that I couldn’t do it, that ‘I can’t do this, I can’t do this,’” she continued.

The prosecution’s questioning of McDonough began Tuesday. Mazzaglia’s defense has not yet begun its cross examination.

McDonough largely was able to retain her composure while on the stand and when exiting and entering the courtroom Tuesday, when prosecutors guided her through questioning about the origins of her relationship with Mazzaglia, whom she met in the spring of 2011 when they were in a theater production in Portsmouth.

Wednesday morning, Judge Steven Houran called for a brief recess as McDonough sobbed in tearful wails that reverberated through the stunned courtroom. She exited the courtroom in tears, her hands near her face.

The horror continued when McDonough resumed her testimony and spent more than the next hour describing her and Mazzaglia’s actions after Marriott’s death.

McDonough said that Mazzaglia came over to her, held out his arms and said “something on the lines of, come on, we’re in this together.”

McDonough said she took that to mean, “if he had to go cover it up, I had to go cover it up.”

Mazzaglia then told her that “we have to figure out a way to get rid of her body,” McDonough said.

In response to questioning from Hinckley, McDonough detailed how they then gathered Marriott’s belongings, turned off her cell phone and their own and wrapped her body in a tarp and a suitcase that wouldn’t entirely close.

“(Mazzaglia) held the suitcase shut while I wrapped the duct tape around,” McDonough said.

They then drove Marriott’s car on a circuitous, uncertain route that ended at Pierce Island in Portsmouth, where McDonough said she and Mazzaglia struggled in “a long process” to push and drag Marriott’s body to a railing that overlooked a cliff.

McDonough said Mazzaglia lifted Marriott’s body and pushed it over the rail, but it landed on rocks, causing them to walk down to a beach and try to reach Marriott’s body from water-level. McDonough said she was the only one able to climb across rocks to Marriott’s body, which she said Mazzaglia asked her to cover in seaweed.

“He said it was too bad we hadn’t tied rocks to her, because then she would have sank,” McDonough said.

She described that moment as “surreal,” as she looked down at someone with whom she had been playing a card game just hours before, worked with at Target in Greenland, and “enjoyed spending time with.”

“Her body was half-floating, covered in seaweed,” McDonough said. “I didn’t even know what to think. I felt so awful, so confused. I had so many conflicting emotions.

“Someone who brought so much joy to people’s lives – and there she was,” McDonough added.

Then, the indescribable loss and tragedy at the center of the murder trial that has shocked and riveted New Hampshire came to light in McDonough's testimony.

“So many things that she’s never going to be able to do,” McDonough said of Marriott. “I’m only 20, but I still look back and think … there’s still so much of life left.

“She never got to get married. She never got to have kids. It’s all because of us, that she never got to live her life,” McDonough continued, sobbing.

“It’s not something we can fix. She can never come back…”

Her voice trailed off, difficult to understand through tears. Judge Steven Houran called the lunch recess.

This time, as McDonough exited the courtroom in halting steps – less than a minute after Mazzaglia, who walked impassively, with no expression discernible on his face – McDonough moved in near silence, her head bowed and her hands clasped in front of her, shuddering.